EL2310 Scientific Programming

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1 Lecture 14: Object Oriented Programming in C++

2 Overview Overview Lecture 14: Object Oriented Programming in C++ Classes (cont d) More on Classes and Members Group presentations

3 Last time Intro to C++ Differences between C and C++ Intro to OOP

4 Today Classes and more on OOP

5 Lecture 14: Object Oriented Programming in C++ Classes (cont d) More on Classes and Members Group presentations

6 C++ Compiler Use g++ instead of gcc Usage and command line options are the same as for gcc Make sure you know how to use make for this part of the course!

7 Declaration of variables You no longer need to declare the variable at the beginning of the function (scope), as was the case for pre C99 Useful rule of thumb: Declare variables close to where they re used. For instance: i only defined within loop for(int i=0;i<n;i++){...} Use specific names for counters, e.g. i,j,k,...

8 Namespaces In C all function share a common namespace This means that there can only be one function for each function name In C++ can be placed in namespaces Syntax: namespace NamespaceName { void fcn();... } To access a function fcn in namespace A A::fcn To avid typing namespace name in every statement: using namespace std

9 Printing to Screen In C++ we use streams for input and output Output is handled with the stream cout and cerr In C: printf("the value is %d\n", value); In C++: cout << "The value is " << value << endl; Just like in C you can format the output in a stream You can use cout.width(10) number of characters for output to fill cout.precision(3) number of digits cout.fill( 0 ) pad with a certain character

10 Getting input from the user Use streams also to get input from console Use the cin stream Ex: int value; cin >> value; If you want to read an entire line, use getline Ex: string line; getline(cin, line); cout << "The input was " << line << endl;

11 References Constrained and safer pointers Compare int a; int a; int *pa = &a; int &ra = a; int *pa = NULL; - *pa = 10; ra = 10; => a==10 int b; int b; pa = &b; - int *pc; - pc = pa; -

12 Passing Arguments by Reference in C++ Declaration: void fcn(int &x); Any changed to x inside fcn will affect the parameter used in the function call Ex: void fcn(int &x) { x = 42; } int main() { int x = 1; fcn(x); cout << "x=" << x << endl; } Will change value of x in the scope of main to 42

13 Dynamic Memory Allocation in C++ In C++ the new and delete operators are used In C we used malloc and free If you allocate an array with new you need to delete with delete [] Ex: int *p = new int[10]; p[0] = 42; delete [] p; Typical mistake, forgotten []

14 The Object-Oriented Paradigm The motivation: We are trying to solve complex problems Complex code with many functions and names Difficult to keep track of all details How can we deal with the complexity? Grouping related things Abstracting things away Creating hierarchies of things This also improves: Code re-use Reliability and debugging

15 Key Concepts of OOP Classes (types) Instances (objects) Methods (Actions) Encapsulation : Data manipulation Inheritance : Code re-use Polymorphism : Code efficiency

16 Classes A class is an extension of a struct : define new data types A class can have both data member and function members (methods) Classes bring together data and operations related to that data Classes works like a namespace

17 Class definition Syntax: class ClassName { public: void fcn(); private: int m X; }; // Do not forget the semicolon!!! m X is a member data void fcn() is a member function public and private are access specifiers

18 Objects Objects are instances of classes Objects correspond to variables of type Class Create a new instance of a class: ClassName variablename;

19 Constructor Constructor is a special kind of method to initialize data members of the class (objects) default constructor (without arguments) + multiple user-defined constructors The constructor has the same name as the class and has no return type class A { public: A() {} };

20 Constructor Example What is the output? class A { public: int m X; int m Y; public: A() { m X=5; m Y=5; } A(int a) { m X=a; m Y=a; } A(int a,int b) { m X=a; m Y=b; } }; A a,aa(10),aaa(10,20); std::cout << a.m X << aa.m X <<aaa.m Y std::endl;

21 Constructor Example What is the output now? class A { public: int m X = 40; int m Y = 40; public: A() { m X=5; m Y=5; } A(int a) { m X=a; m Y=a; } A(int a,int b) { m X=a; m Y=b; } }; A a,aa(10),aaa(10,20); std::cout << a.m X << aa.m X <<aaa.m Y std::endl;

22 Classes (cont d) Lecture 14: Object Oriented Programming in C++ Classes (cont d) More on Classes and Members Group presentations

23 Classes (cont d) Destructor When an object is deleted the destructor is called to clean up things For instance, to free up dynamically allocated memory There is only 1 destructor If not declared, a default one is used which will not free up dynamic memory Syntax: ClassName(); Class A { public: A(); // Constructor Ã(); // Destructor... };

24 Classes (cont d) Source and header file Normally you split the definition from the declaration like in C The definition goes into the header file.h The declaration goes into the source file.cpp Header file ex: class A{ public: A(); private: int m X; }; Source file ex: #include "A.h" A::A():m X(0)

25 Classes (cont d) Let s look at some examples

26 Classes (cont d) Task 1 Implement a class the defines a Car Should have a member variable for number of wheels Should have methods to get the number of wheels Write program that instantiate a Car and print number of wheels

27 Classes (cont d) Task 2 Write a class Complex for a complex number Provide 3 constructors default - which should create a complex number with value 0 having one argument - should create a real value having two arguments - should create a complex number with real and imaginary part

28 More on Classes and Members Lecture 14: Object Oriented Programming in C++ Classes (cont d) More on Classes and Members Group presentations

29 More on Classes and Members this pointer Inside class methods you can refer to the object with this pointer The this pointer cannot be assigned (done automatically) Example use in our toy constructor example.

30 More on Classes and Members const Can have const function arguments Ex: void fcn(const string &s); Pass the string as a reference into the function but commit to not change it For classes this can be used to commit to not change an object as well Ex: void fcn(int arg) const; The function fcn commits to not change anything (its class members) in the object it belongs to

31 More on Classes and Members Static members Members (both functions and data) can be declared static A static member is the same across all objects; it s a member of the class, not any single object That is all instantiated objects share the same static member You can use a static class member without instantiating any object You need to define static data member Ex: (in source file) int A::m Counter = 0; if m Counter is a static data member of class A Static methods can only use static data members!

32 More on Classes and Members Task 3 Start from the Complex class from last time Add a static int member Every time a new complex number is created the static variable should be incremented Implement the member function Complex& add(const Complex &c); which should add c to the object How does the number of created objects change if we change the function to Complex& add(complex c);

33 Group presentations Lecture 14: Object Oriented Programming in C++ Classes (cont d) More on Classes and Members Group presentations

34 Group presentations Presentation today - Group 11 How to interface between Matlab and C using MEX Calling Matlab functions in C program Calling C functions in Matlab

35 Group presentations Presentation on 12th Oct (Monday) Group 10 - How to optimize C code. Explain with examples Group 12 - Introduce Genetic Algorithms (GA). Implement a GA solution for a problem in C++, e.g., Traveling Salesman Problem

36 Group presentations Presentation on 14th Oct (Wednesday) Huffman Coding for compression Implement it in C++

37 Group presentations Presentation on 15th Oct (Thursday) Group 14: Expectation-Maximization (EM) algorithm Monte Carlo Sampling for inference and approximation. Implement an example in C++ Group 15: Introduce Multi-threading Show some implemented examples in C++

38 Group presentations The C++ project Will be announced tomorrow Deadline: Monday Lab/help session: Friday , 1-3:00pm, Room 22:an, Teknikringen 14 Reminder: C project deadline tomorrow

EL2310 Scientific Programming

EL2310 Scientific Programming (pronobis@kth.se) Overview Overview Wrap Up Introduction to Object Oriented Paradigm More on and Members Operator Overloading Last time Intro to C++ Differences between C and C++ Intro to OOP Today Object